Several materials are used as substrate material and are under further development for the production of thin film flexible products. There exist several advantages to use flexible substrate materials, one being the possibility of roll-to-roll production processes, which lead to a more cost efficient production compared to batch type processes. Moreover, the thin film flexible products will have several technical advantages, for instance, they may be folded or rolled into compact packages and they may be used for making light weight products, which are required for portable, spatial and military applications. The common materials used for thin film flexible products include, e.g., plastic foils such as polyamide, metallic strips or foils such as stainless steel, titanium, copper, molybdenum, aluminum and nickel foils, bearing in mind that they all have to fulfil certain criteria. Thus, the substrate material should be thermally resistant in order to withstand further process steps in the production of thin film products, and this may include heat treatments at elevated temperatures under corrosive atmosphere. However, all the electrical conducting metallic strip materials used as substrate material for the production of thin film flexible products may need to be electrically insulated, if modules with integrated series connections are to be produced. Conventionally, the conducting metallic strip or foil material is coated with oxides such as aluminum oxide or silicon oxide, which are good electrical insulating oxides. However, these oxide layers usually have a large mismatch in the thermal expansion compared with the underlying metallic substrate. Thus, it is essential that the thermal expansion coefficient (TEC) of the substrate material should be as close as possible to the TEC of the electrically insulating metal oxide layer(s), in order to avoid thermal cracking or spallation of the insulating metal oxide layer.
The mismatch in thermal expansion (MTE) can be defined as follows: (TECss−TECox)/TECss where the TECss is the thermal expansion of the metallic strip substrate and TECox is the thermal expansion of the metal oxide layer.
Ferritic stainless steels and yttrium-stabilized zirconia (abbreviated YSZ) are two materials commonly used in the fabrication of Solid Oxide Fuel Cells because these two materials have an extremely low mismatch in their thermal expansion. However, the YSZ in Solid Oxide Fuel Cells are not in the form of a thin coating but in the form of a thin plate, which is used as an electrolyte.
Common conventional solutions are:                Depositing the electrically conducting back contact directly onto the metallic strip;        Depositing insulating layers such as aluminum oxide, silicon oxide and silicon nitride onto metal strips,        
Both these solutions have their respective disadvantages. If the conducting back contact is directly deposited onto the flexible metal strip substrate, then it will limit the production of modules with integrated series connections. Furthermore, the material often used as insulating layers, such as Al2O3, Si4N3, SiOx or SiO2, have a larger TEC mismatch with the underlying metallic strip, which may lead to the formation of cracks and pinholes due to heating during the following process steps. In table 1 the thermal expansion coefficients of some insulating materials and some steels have been summarized. Using the values of the thermal expansions given in Table 1, it is possible to calculate the thermal expansion mismatch (MTE) between the metallic strip substrate and the insulating coating. In Table 2 the MTE values between some of the steels and the listed insulating materials are given.
It is therefore a primary object of the present invention to provide a zirconium oxide coated metallic strip product, where the oxide layer and the metallic strip have a very low mismatch in thermal expansion, said product being suitable as a substrate material for the production of thin film flexible products, such as flexible Cu(In; Ga)Se2 (abbreviated CIGS) solar cells and solid state thin film batteries.
Yet another object of the present invention is to provide a flexible substrate for thin film products that is inexpensive and which may be produced in a continuous roll-to-roll process.
These and other objects have been attained in a surprising manner by creating a coated metallic strip product with the features according to the characterizing clause of claim 1. Further preferred embodiments are defined in the dependent claims.